1. Field Of The Invention
The present invention relates to a method for filling prepared dental cavities in a manner which substantially reduces corrosion at the interface of the tooth-amalgam restorations. More particularly, the present invention relates to a cavity lining material containing an adhesive which not only improves the tenacity of the bonding of a dental restoration to cavity surfaces, but also is effective in substantially reducing the susceptibility of amalgam fillings to corrosion.
2. Description Of The Prior Art
Normally, dental cavities are filled with a silver-tin amalgam. With time, however, the amalgam restoration degenerates by corrosion and, as a result, it must be replaced.
Silver-tin dental amalgam is produced by triturating or mixing about equal quantities of the powdered silver-tin alloy, principally Ag.sub.3 Sn, and Hg. The mixture, when placed in a cavity, hardens to form several alloy phases. Of the phases which form, the two which predominate are a silver-mercury phase, Ag.sub.2 Hg.sub.3 (.gamma..sub.1) and a tin-mercury phase, Sn.sub.7-8 Hg (.gamma..sub.2). Over a period of time, the tin from the .gamma..sub.2 phase dissolves or corrodes because tin is less noble (more easily oxidized) than either Ag or Hg and because of factors such as differential aeration. This corrosion, of course, reduces the strength of the amalgam restoration, causes discolorization of the tooth, and contributes to marginal discrepancies and eventual partial extrusion of the restoration from the cavity.
Differential aeration causes the corrosion process (i.e., tin dissolution) to occur most rapidly in areas of lower oxygen concentration. Thus, the primary area of corrosion is at the restoration-tooth interface. Corrosion of the margins (that part of the restoration-tooth interface that is exposed to saliva) is especially critical as such corrosion contributes to recurrent decay. Thus, the corrosion process usually occurs from the inside-out, which weakens the restoration.
Differential aeration is a common oxidative phenomenon. For instance, it is the mechanism which causes rust to spread circumferentially under the paint around a pin-hole blemish on painted steel. In a dental restoration, the first step of differential aeration is the reduction of oxygen on the unexposed restoration-tooth interface. Mercury and silver are relatively inert in comparison to tin and only participate as sites for oxygen reduction. The anodic oxidation (corrosion) of the surface tin atoms causes an electron transfer to occur from the tin atoms at the surface of the amalgam restoration. These electrons are conducted to the exposed (oxygen rich) surface of the amalgam through the amalgam bulk, resulting in the cathodic reduction of oxygen atoms. The net effect is that the oxidation of tin atoms, which occurs on the unexposed (to air) amalgam surface, results in the generation of tin ions within the interface. In addition, the presence of the tin ions in the areas adjacent to the cavity surfaces promotes further deterioration of the dental restoration by the precipitation of tin hydroxides which cause the area to become more acid.
One of the reasons that the silver-tin (Ag.sub.3 Sn) alloy is used is that the resulting amalgam is reasonably strong and expands or contracts very little upon hardening. Because tin is less noble than silver or mercury, and because the atomic proportions of tin and mercury in the .gamma..sub.2 phase (Sn.sub.7-8 Hg) are unfavorable, it is essentially the only part of the amalgam that corrodes. Hence, the corrosion of dental amalgam restorations seems to be a result of differential aeration, and of the presence of tin in the .gamma..sub.2 phase.
The inventors in an earlier patent application have described a method for preparing the surface of dental cavities so that the dental amalgam which is used to fill the cavity exhibits a substantially reduced tendency to deteriorate over long periods of time. The method involves the application of a liner of a metal such as silver which will amalgamate with mercury to the cavity surfaces prior to filling the cavity proper with dental amalgam. Mercury diffuses from the amalgam bulk into the metal liner so that a metal alloy region is formed between the amalgam and cavity surfaces which is free of tin atoms. Differential aeration, therefore, cannot occur in this region and consequently, dental restorations prepared in this manner possess longer effective lifetimes than conventional dental restorations.
A continuing problem with dental restorations is, that despite the improvements described above concerning the improved stability of dental restorations, a need continues to exist for a method of improving the bonding strength of dental restorations to cavity surfaces while at the same time exhibiting substantial corrosion resistance.
One prior art technique of attempting to improve the bonding strength between dental restorations and cavity surfaces as shown by Saffir in U.S. Pat. No. 3,513,123 involves the application of a cavity lining composition consisting of an epoxy resin and a dental alloy or amalgam prior to filling of the cavity with an amalgam. While the lining material may promote adhesion between the applied dental restoration and the cavity surfaces, the overall restoration exhibits the same susceptibility to corrosion as conventional restorations.
Another technique of attempting to improve the adhesion of dental restorations to cavity surfaces as described in Union Broach (1972) involves the application of a silver lining to cavity surfaces prior to filling the cavity with dental amalgam from a composition of silver in a eugenol base. While the silver lining improves adhesion of the dental amalgam to the cavity surfaces, the resistance of the filling to deterioration by oxidation is not improved over conventional dental restorations because the relatively high amounts of eugenol in the composition interfere with the effective bonding of the amalgam with the liner.
Yet another prior art method is known for the filling of pits and fissures in teeth as disclosed by Takeuchi, U.S. Pat. No. 3,518,762 in which a monomeric lower alkyl .alpha.-cyanoacrylate is first applied to the pit or fissure and thereafter, microfine particles of a material such as gold, silver, tin or the like which may be in admixture with or coated with a lower alkyl methacrylate polymer are applied to the pit or fissure to complete the filling. The composition, however, is used to prevent carious formation in teeth and is not used in the preparation of dental restorations.
A need, therefore, continues to exist for a technique of both improving the adhesion of dental amalgams in prepared cavities as well as improving the resistance of the same to corrosion.